Most high speed communication and navigation systems rely on accurate clock generation at both the transmitter and receiver. Often the local oscillator in the receiver needs to have an adjustable frequency in order to tune to the frequency of the incoming signal. This tuning is typically done with a Phase Locked Loop or PLL which is usually implemented using electronic circuitry. Most PLL implementations in turn employ a Voltage Controlled Oscillator or VCO to provide a variable frequency within a feedback loop. The VCO when implemented in semiconductor processing methods is composed of a fixed inductor and a variable capacitor (or varactor) forming an adjustable LC tank circuit.
The LC tank circuit operates in the electronic domain and is therefore subject to electronic noise such as fluctuations in the power supply, thermal noise, and other noise sources.
While VCOs have been developed and refined to have excellent specifications, improvements in noise characteristics are they suffer from the aforementioned noise which reduces the overall spectral purity of the output signal.
Other than electron based oscillators, optical and electro-optical methods have been developed. Many of these system base the oscillatory period on the frequency of laser light. The Brillouin Opto-Electronic Oscillator described in [2] for example derives its frequency from the beat frequency between the laser signal and the Brillouin signal. The tunable optical resonator described in [3] utilizes piezoelectric effect as well as thermal expansion to adjust the optical path length and thus change the output frequency.